Differential signaling is a data transfer technique widely used in digital and analog data interfaces due to its high noise immunity during data transmission. An example using a differential signaling interface is universal serial bus (USB), which is used to interconnect digital devices such as personal computers, printers, and scanners, as well as digital recording devices including digital versatile discs (DVDs) and digital video cameras. Currently, USB interfaces are increasingly used for common, hand-held digital devices such as mobile phones and portable media players.
In a differential signaling system, a differential transmitter transmits a differential signal to a differential receiver over a communication link, which is physically a pair of wires carrying complementary input voltages Vin+ and Vin−. Upon receiving the differential signal, the receiver compares Vin+ and Vin− to output a “0” or “1” depending on whether or not the two voltages differ in amplitude, thus extracting intended data from the incoming differential signal.
Typically, such a differential signaling system incorporates a squelch detector to detect a state of communication link at the receiver end. A squelch detector detects a difference between Vin+ and Vin− to output a squelch signal, indicating a squelch state when the voltage difference does not exceed a given threshold, and an unsquelch state when the voltage difference does exceed the given threshold. For example, when the threshold value is set at 100 mV, as is defined by USB specifications, a squelch detector detects a squelch state for a difference of 50 mV and an unsquelch state for a difference of 150 mV. Such a squelch signal is used to determine whether the differential signal, which may normally exhibit a given level of voltage difference, is present or not on the communication link. Thus, the receiver invalidates the differential signal when the communication link is in a squelch state.
Currently, some differential signaling systems use low voltage and reduced differential swings to achieve high data transfer rates. For example, a serial data interface implementing USB 2.0 can operate at a transfer rate of up to 480 Mbps in high-speed mode. Such high-speed differential signaling, where it becomes difficult to distinguish signal components from noise, requires a stable and secure system to ensure reliable data transmission, such as a receiver with a high gain amplifier and a squelch detector with increased stability.
Various schemes have been proposed for enhancing data transmission reliability. For example, one conventional method for stable squelch detection uses a reduced number of amplifiers in the detector circuit. Such a circuit design may provide stability in squelch detection without increasing power consumption and manufacturing cost, but involves a complicated structure which may sacrifice efficiency in the circuit.
Another conventional method introduces a squelch detector with a control circuit that controls a squelch signal to stably provide squelch detection. However, this conventional method may be impractical when used with a serial data interface such as USB 2.0 interfaces.
Both of these conventional approaches are made to enhance performance of a squelch detector, which may have limited effectiveness in providing reliable data transmission. Thus, there is still a need for a useful, non-complicated system for reliable data transmission that can meet requirements of high-speed differential signaling.